The present application claims priority to Japanese Application No. P11-232786 filed Aug. 19, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a radio wave absorber, and in particular to a radio wave absorber comprising a mixed material having a magnetic material grain and a resin material or a ceramic material.
2. Description of Related Art
Radio wave absorbers have been used for electric devices, communication apparatuses or the like in order to stabilize functions thereof by absorbing radio waves coming in externally as a disturbance or emitted internally as a leakage. An example of a radio wave absorber already put into practical use is a composite material of grain and resin, where the grain is a spinel ferrite sintered compact, hexagonal ferrite sintered compact or flaky metal soft magnetic material. Conventional radio wave absorbers can absorb radio waves having a frequency of several MHz to several GHz band.
Material parameters expressing characteristics of such a radio wave absorber are complex permittivity xcex5 and complex permeability xcexc in a high frequency range. As for a radio wave absorber using a magnetic material, xcexcxe2x80x3 as an imaginery component of complex permeability xcexc (=xcexcxe2x80x2xe2x88x92jxcexcxe2x80x3) relates to the radio wave absorbing characteristics.
FIG. 8 shows complex permeability xcexc (=xcexcxe2x80x2xe2x88x92jxcexcxe2x80x3) of spinel ferrites. In the figure, the real component xcexcxe2x80x2 and imaginery component xcexcxe2x80x3 are shown for each of three types of ferrites xe2x80x9caxe2x80x9d to xe2x80x9ccxe2x80x9d. As is clear from the figure, the real component xcexcxe2x80x2 descends in a frequency range higher than a certain value and the imaginery component xcexcxe2x80x3 reaches a maximum value at a resonant frequency fr which is slightly higher than the above certain value. Higher xcexcxe2x80x3 corresponds with better absorption due to larger energy loss.
Spinal ferrite with a higher permeability, however, tends to resonate at a lower frequency, and this has prevented the material from being used at a high frequency such as in a GHz range (snake limit). This is expressed by the equation (1) below, which indicates that a product of resonant frequency and permeability is contant:                                           f            r                    ⁢                      xe2x80x83                    ⁢                      (                                          μ                xe2x80x2                            -              1                        )                          =                              γ                          3              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              μ                0                                              ⁢                      xe2x80x83                    ⁢                      I            s                                              (        1        )            
where, fr is the resonant frequency, xcexcxe2x80x2 is the real part of the permeability, r is the gyro-magnetic constant, xcexc0 is the permeability of vacuum and Is is the saturation magnetization.
To solve the problem of spinel ferrite exhibiting only a low level of absorption in the high frequency range, Y-type and z-type ferrites have been put into practical use, where the crystal structures of the ferrites belong to the hexagonal system and exhibit in-plane magnetic anisotropy. The use of such ferrites is based on the hexagonal ferrite having a high permeability due to a small in-plane magnetic anisotropy and requiring a larger anisotropic energy to orient the direction of magnetization toward the direction normal to the major plane, so that such ferrite can resonate at a higher frequency range than the spinel ferrite can.
However, the resonance level has been limited to several GHz even with such a hexagonal ferrite. The product of the resonant frequency fr and the permeability xcexcxe2x80x2 in this case is expressed by the equation (2) below:                                           f            r                    ⁢                      xe2x80x83                    ⁢                      (                                          μ                xe2x80x2                            -              1                        )                          =                                            γ              ·                              I                s                                                    3              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              μ                0                                              ⁢                      xe2x80x83                    ⁢                                                    H                A2                                            H                A1                                                                        (        2        )            
where, HA1 is the in-plane anisotropy an HA2 is the anisotropy from the in-plane to xe2x80x9ccxe2x80x9d axis direction (see FIG. 1).
It is now noticed by comparing the equations (1) and (2), that the equation (2) additionally has a square-root term. The hexagonal ferrite generally has a value of 1 or above for the square-root term as shown by the relation below:                                                         H              A2                                      H              A1                                      ≥        1                            (        3        )            
This is why the hexagonal ferrite can retain a higher permeability in a higher frequency range. The available frequency range of the hexagonal ferrite is, however, limited by its saturation magnetization of about 0.5 T, and a material available at a frequency range of several GHz or above is still unknown. Accordingly the available frequency range of the radio wave absorber has also been limited to several GHz.
Considering the foregoing situation, it is therefore an object of the present invention to provide a magnetic material exhibiting a high permeability at a high frequency range and to provide a radio wave absorber exhibiting an excellent radio wave absorbing property by using such a magnetic material.
To achieve the foregoing object, the present invention provides a radio wave absorber comprising a mixed material having a magnetic material grain, and one of either a resin material or a ceramic material, wherein said magnetic material grain is in the shape of a disc.
As a result of the disc-shaped magnetic material grain, the frequency limit can be raised up to a level of several GHz or above, and thus a radio wave absorber exhibiting a high permeability at a high frequency range can be obtained.
Conventional magnetic material grains were obtained by pulverizing magnetic material by proper means and had irregular shapes, so that the snake limit could not be overcome. As means for overcoming such a limit, Y-type and Z-type ferrites are commercialized by Royal Philips Electronics. These ferrites have in-plane magnetic anisotropy unlike the spinel ferrite.
To obtain a higher permeability at a higher frequency range than the Y-type and Z-type ferrite material can, raising the saturation magnetization Is will be successful, which is obvious from the equation (2) above. The ferrite generally has a saturation magnetization of 0.3 T to 0.5 T. In contrast, a metal magnetic material, more specifically, pure iron has such a value of 2.2 T, iron-cobalt alloy (Permendule) 2.4 T, and iron nitrides compound 2.8 T. It is thus understood that developing a metal magnetic material having in-plane magnetic anisotropy will successfully overcome the snake limit and ensure the operation at a higher frequency range than Y-type or Z-type oxide magnetic material having in-plane magnetic anisotropy can.
However, a metal soft magnetic material having in-plane magnetic anisotropy has, not been found. In the present invention, a disc-shaped metal magnetic material was prepared so as to attain a magnetic anisotropy within the in-plane direction, and thus a high permeability at a high frequency range was achieved while retaining a high saturation magnetization. By virtue of such a high permeability, the present invention is successful in obtaining a radio wave absorber having a more advanced absorption property than the conventional one.
According to the present invention, the frequency limit of the magnetic material was successfully raised as high as several GHz or above by fabricating the grain thereof in a disc shape, so that a radio wave absorber exhibiting a high permeability at a high frequency range can be obtained. This allows a radio wave absorber to operate at as high as 10 GHz or above, which has previously been available only in a frequency range as high as 2 to 3 GHz, and to prevent radio wave failure over a wide frequency range. The high permeability achieved herein also allows a reduction in thickness of the radio wave absorber, so that a small-sized and compact radio wave absorber can be fabricated.